Flat displays for information apparatuses have become widespread remarkably in recent years. Among them, a liquid crystal display performs on/off control of light of a backlight unit by using the optical shuttering function of liquid crystal and obtains color by using color filters. On the contrary, an organic EL display (or an organic LED display) has not only an advantage such that the viewing angle is wide because each pixel emits light individually (that is, self-emits light), but also many advantages such that the display can be made thin and formed on a flexible substrate because there is no backlight unit required. For this reason, the organic EL display is expected as a next-generation display.
Methods for driving these display panels can be roughly classified into two groups. The first drive method is called passive matrix type (or duty drive method, simple matrix method). This is a method in which stripe electrodes are combined in rows and columns like a matrix so that a pixel located at an intersection of a certain row and column is made to emit light by a drive signal applied to a row electrode and column electrode. The signal for light emission control is generally scanned in time series on each row in the row direction and applied to respective columns simultaneously on the same row. This is a method in which each pixel is generally provided with no active element so that each pixel is controlled to emit light only in the duty period of each row in the scanning period of the row.
The second drive method is called active matrix type in which each pixel is provided with a switching element so that the pixel can emit light in the scanning period of a row. For example, assume the case where the whole surface of a panel having 100 rows and 150 columns is made to emit light at display luminance of 100 Cd/m2. In this case, in the active matrix type, each pixel can be made to emit light at 100 Cd/m2 simply if the area ratio of each pixel and various kinds of loss are not considered because each pixel fundamentally always emits light. However, in the passive matrix type, when the same display luminance is to be obtained, the light emission luminance in the light-emitting time need to be set at 10000 Cd/m2 which is 100 times higher because the duty ratio for driving each pixel is 1/100 and only the duty period (selection period) is the light emitting time.
Here, the light emission luminance can be increased when a current flowing in a light-emitting element is increased. However, for example, in an organic EL light-emitting element, it is known that light-emitting efficiency decreases as the current increases. In comparison between the active matrix type drive method and the passive matrix drive method at the same display luminance, power consumption in the passive matrix type becomes relatively large because of reduction of this efficiency. Moreover, increase of the current flowing in the organic EL element has a disadvantage that deterioration of material is apt to be caused by heating or the like and the life of the display unit is shortened. On the other hand, if the maximum current is limited from the viewpoints of efficiency and life, the necessity to elongate the light-emitting period occurs in order to obtain the same display luminance. However, the elongation of the light-emitting period lends itself to limitation of display capacity (number of drive lines) because the duty ratio for deciding the light-emitting time in the passive matrix type drive method is the reciprocal of the number of rows in the panel. From these points, it was necessary to use the active matrix type drive method in order to achieve a large-area high-definition panel. As for a basic circuit for general active matrix drive, a method using thin-film transistors as switching elements is known.
In the active matrix type drive method adapted to the large area and high definition, a thin-film transistor (TFT) using polysilicon as a switching element of a pixel is used most widely. However, since polysilicon is a polycrystal, there is a large problem such that in-plane uniformity is poor particularly in a current drive type display such as an organic EL display. Further, in amorphous silicon, in-plane uniformity is good because of the amorphous structure but there arises a problem that the mobility thereof is at most about 1 cm2/Vs which is not enough to drive the organic EL element.
To cope with such various problems in the background-art display panels, an amorphous oxide semiconductor has been recently proposed as a semiconductor material used for the thin-film transistor.
For example, a field-effect transistor (FET) using an InGaZnO4 film produced as a semiconductor at room temperature has been disclosed in Non-Patent Document 1. In this, it has been disclosed that the InGaZnO4 film as an active layer has an amorphous structure, the field-effect mobility is 8 cm2/Vs and the mobility measured based on Hall effect is about 12 cm2/Vs.
Further, in Patent Document 1, it has been disclosed that positive ions are injected into an amorphous oxide film represented by ZnxMyInzO(x+3y/2+3z/2) (in which M is Al or Ga, x/y is 0.2 to 12, and z/y is 0.4 to 1.4) to thereby exhibit electrical conducting property. Non-Patent Document 1: Kenji Nomura and other five, “Room-Temperature Fabrication of Transparent Flexible Thin Film Transistors Using Amorphous Oxide Semiconductors”, Nature, vol. 432, pp. 488-492, November 2004
Patent Document 1: JP-A-2000-44236